I have a curved pipe with a half circular cross-section with a symmetry plane defined in the plane of the curvature. The y-direction is normal to the symmetry boundary and as expected the nodal values of the v-component of velocity are zero at the symmetry boundary. Strangely, the velocity gradient dv/dy does not equal to zero at the cell-center or nodal locations on the symmetry boundary. In addition, the pressure gradients are zero in the symmetry plane. Based on the description in the Fluent manual I expected the gradients d/dy to also be zero.

I have a curved pipe with a half circular cross-section with a symmetry plane defined in the plane of the curvature. The y-direction is normal to the symmetry boundary and as expected the nodal values of the v-component of velocity are zero at the symmetry boundary. Strangely, the velocity gradient dv/dy does not equal to zero at the cell-center or nodal locations on the symmetry boundary. In addition, the pressure gradients are zero in the symmetry plane. Based on the description in the Fluent manual I expected the gradients d/dy to also be zero.

The secondary flow structure has been shown in the literature to be symmetric across the horizontal centerline (from the inner wall to the outer wall) of the pipe cross-section. This is why I used the symmetry plane along the horizontal centerline. The results show the expected secondary flow structure within the cross-section when I use the symmetry plane but the velocity gradients at the symmetry plane are not zero for some reason.